Workpiece feature-specific illumination is often necessary in precision machine vision inspection systems to accentuate an edge or other feature in an inspection image. Many such systems including lighting systems are capable of providing relatively focused coaxial illumination through a camera or microscope lens, “stage light” from behind a workpiece to create an accurate silhouette image, and/or diffuse illumination. Some lighting systems include arrangements that are capable of projecting light onto the workpiece at an adjustable angle of incidence relative to an axis which is normal to the nominal object plane that is imaged. In many conventional vision systems, the axis normal to the nominal object plane is parallel to, or coincides with, the optical axis of the vision system. Light projected at a selected angular range and/or angle of incidence between 0° and 90° may improve the contrast of edges in the image and/or may more clearly illuminate textured surfaces. Typically, such light sources have a selectable range for the angle of incidence varying between approximately 10° and 70°. Such a range is relatively broad so as to have the capability to enhance image contrast for a variety of types of workpiece features.
Enhanced image contrast for a feature is important because image processing algorithms that perform edge-finding and surface autofocus operations are often designed to detect locations that are associated with maximum grayscale gradients in a inspection image. Thus, grayscale gradients are of particular significance in workpiece inspection images, and these “gradient signals” are enhanced when the image contrast is enhanced.
Some lighting systems can also adjust or select the radial direction of the lighting about an optical axis, for example, by using lighting elements arranged around the optical axis in a “ring light” configuration that includes addressable sectors or quadrants. The field of view of a camera can be illuminated by any combination of sectors or quadrants of such a generally ring-shaped lighting system. The intensity level and/or color content of the light source can be coordinated with the angle of incidence and the radial direction of the light source to optimize the illumination of a workpiece edge.
In one type of lighting system, light emitting diodes (LEDs) are arranged in an annular pattern to surround the optical axis of the vision system. An exemplary lighting system of this type is disclosed in U.S. Pat. No. 5,897,195, issued to Choate (hereinafter “the '195 patent”). The '195 patent discloses an oblique LED illuminator device made from a cylindrical or truncated-conical array of LEDs. The array of LEDs produces collimated light beams and directs them to the inclined surfaces of an annular-shaped Fresnel-like diffuser, which is coaxially arranged radially inward of the array of LEDs. The Fresnel-like diffuser refracts and directs LED light beams onto the surface of a workpiece at various angles of incidence. The Fresnel-like diffuser includes a plurality of annular, prism-shaped projections which differ in shape depending upon the desired angle of incidence. To create a beam of light with a particular angular range and/or angle of incidence, a light beam is emitted from particular LEDs (e.g., one or more LEDs) to the associated prism-shaped projection(s) within the Fresnel-like diffuser, which redirects the light beam onto the workpiece with the particular angular range and/or angle of incidence.
Strobe lighting is increasingly used in precision machine vision inspection systems in order to provide accurate unblurred images while moving the workpiece continuously, thereby improving inspection throughput. While the lighting system of the '195 patent allows for fast switching among various LEDs and hence among various angles of incidence, the relatively low optical energy output of LED's limits the illumination intensity, which extends the exposure time required for a given image exposure (e.g., to tens or hundreds of microseconds, or more). Longer image exposure times generally require slower workpiece motion in order to limit workpiece blur to an acceptable level. Therefore, the system of the '195 patent limits inspection throughput because it limits allowable workpiece motion speeds.
In another type of ring lighting system, LEDs and associated optical elements are selectively moved to achieve a desired angle of incidence. An example of this type of system is disclosed in U.S. Pat. No. 6,857,762, issued to Shimokawa et al. (hereinafter “the '762 patent”), which is incorporated herein by reference. The '762 patent discloses LEDs that are arranged generally annularly around an optical axis of the vision system. An annular reflector mirror is coaxially arranged radially outward of the annular LEDs, such that the light from the LEDs is directed at the reflector mirror and is reflected therefrom toward a focal point on the imaging plane at a specific angle of incidence. The reflector mirror has a convex surface having a varying radius of curvature, and is movable relative to the LEDs along the optical axis. Thus, a user can selectively move the reflector mirror relative to the LEDs such that the light from the LEDs will be reflected from a different position along the varying radius of curvature of the reflector mirror so as to illuminate a focal point on the imaging plane at a different angle of incidence. In this arrangement, the LEDs may be replaced with high-intensity light sources such as fiber optic cables used with a halogen lamp. Specifically, a number of fiber optic cables may be arranged such that first ends of the fiber optic cables receive light from a high-intensity light source, such as a halogen lamp, while the second ends of the fiber optic cables are arranged in an annular shape around the optical axis. The fiber optic cables, or sets of the fiber optic cables, can be individually controlled to project the high-intensity light from the light source onto the field of view of a camera at a desired angle of incidence based on a selective movement of the annular reflector mirror. While such a lighting system allows for adjusting the angle of incidence of a high-intensity illumination source, due to the mechanical movement of various parts involved, this adjustment may take tens or hundreds of milliseconds or more. Thus, adjustment of an angle of incidence cannot be performed as fast as in the case of the '195 patent, described above, which includes no moving parts. Therefore, the system of the '762 patent limits inspection throughput because the workpiece may have to be delayed at (or between) workpiece feature imaging positions while waiting for the lighting system configuration to be reconfigured to the particular configuration to be used for the next image.
A need exists for an illumination system configuration for use in a precision machine vision inspection system, which allows for inspection throughput increases by providing both very fast adjustment of the angular range and/or nominal angle of incidence of illumination, as well as the use of a high-intensity light source, such as a halogen lamp.